1. Field of the Invention
The present invention relates to electro-optical scanners for reading a one or two-dimensional bar code symbology, and more particularly, to a hand held scanner capable of specular imaging in very low contrast symbology environments.
2. Description of Related Art
Optical imaging systems are commonly used to decipher data symbols printed on objects in order to identify the objects or to capture information relating to the object. A bar code symbol represents a common one-dimensional form of symbology, and comprises a pattern of vertical bars of various widths separated by spaces of various widths. Because the bar and space elements have different light reflecting characteristics, a reader can convert the symbology into an electrical signal by analyzing the light reflected from the symbol. The electrical signal can then be decoded to provide an alphanumeric representation of the symbol which identifies the object. Bar code symbols of this nature are now in common usage in various applications, such as inventory control, point of sale identification, or logistical tracking systems.
Since the conventional one-dimensional symbology requires a relatively large amount of space to convey a correspondingly small amount of data, so-called two-dimensional bar code symbologies have been developed. A two-dimensional symbology may comprise a matrix that occupies a uniform amount of space having a generally rectangular or square shape. Instead of bars and spaces, round or square marks disposed at particular rows and columns of the matrix correspond to the information being conveyed. As a result, a two-dimensional matrix symbology can compress significantly more data into a given volume of space than a conventional one-dimensional bar code.
As known in the art, the two-dimensional symbols are read by scanners that convert the symbols into pixel information, such as described in U.S. Pat. No. 4,988,852 issued to Krishnan. The pixel information is in turn deciphered into the alphanumeric information represented by the symbol. Such scanners often utilize charge-coupled device (CCD) technology to convert optical information from the symbol into an electrical signal representation of the matrix. A light source illuminates the symbol, and diffuse light reflected off the symbol is focused onto the surface of the CCD device. The two-dimensional scanners may be provided in a portable device so that they can be brought into close proximity with the item on which the symbol is placed, such as disclosed in U.S. Pat. No. 5,378,883 issued to Batterman et al. The scanner may also be provided in a fixed-position device that images items as they pass by, such as on a production line.
In one particular application of a two-dimensional symbology, a small symbol can be placed directly onto items having low surface area, such as electronic components. The two-dimensional symbol could be formed directly onto the ceramic or plastic package of the electronic components by laser etching or other precision machining process. Since a two-dimensional symbology can compress fifty or more characters of data within a relatively small dimensional space, the symbol can store a unique identifier code for the component, including such information as serial number, lot number, batch number, model number, and/or customer code. The symbols can be used to automate the manufacturing or testing processes, and may also enable manufacturers to protect against component theft or forgery.
Another application of two-dimensional symbology is in the marking of silicon wafers used in manufacturing integrated circuits. As known in the art, a lithographic process is used to form onto a silicon substrate the detailed features associated with individual components that collectively comprise an integrated circuit. In the lithographic process, a desired circuit layout is projected onto the substrate through a mask or reticle. The substrate is coated with a resist material that is sensitive to the particular wavelengths of light that pass to the substrate through transparent regions of the mask. The resist material undergoes a chemical transformation at the portions exposed to light, permitting selective removal of the resist material leaving an image of the circuit layout printed on the substrate. Using the same lithographic process, a two-dimensional symbol can be printed directly onto the substrate. Ideally, the two-dimensional symbol would be disposed in an unused region of the wafer or close to an edge of the wafer.
A significant drawback of such etched, printed or machined symbols is that they have very low contrast and, as a result, are difficult to image. Since the symbol characters are formed by shallow cuts into the surface of a component or substrate, there is little color difference between the characters and the remaining uncut surface area. Also, electronic components often have a dull black finish that tends to further obscure the symbol characters. The characters can only be distinguished by the slight difference in shade due to shadows which form in the etched regions. Another factor to consider in the imaging of such low contrast symbols onto silicon wafers is the delicate nature of the wafers and the undesirability of having a portion of the scanner come into physical contact with the substrate surface of the wafers.
U.S. Pat. No. 5,393,967 issued to Rice et al. discloses a system for reading symbols encoded as a low contrast relief pattern. The disclosed system sweeps or scans a line of light across the relief pattern at a first angle and views light reflected off of the relief pattern at a second angle. The system disclosed in the Rice patent necessarily utilizes relatively complex mechanical and optical systems, which presents a drawback to the system.
Accordingly, a need exists for a method and apparatus for making images of low contrast symbols. Such a method and apparatus would be highly desirable in the manufacture of electronic components and silicon wafers.
In accordance with the teachings of the present invention, an apparatus and method that permits specular imaging of low contrast symbols is provided.
Particularly, the imaging device of the present invention includes a light source for directing light onto a target that includes a low contrast symbol. An imaging element receives specular light reflected off of the target and creates an image of the target therefrom that is stored in a data memory. A light level detector may also be used to determine the intensity level of the light received by the imaging element, and when the intensity level exceeds a predetermined threshold, a controller causes the image data created by the imaging element to be stored in the data memory.
In an embodiment of the invention, a light wand is provided with the light source and imaging element. The light wand is adapted to read low contrast bar code symbols disposed on a substrate material using specular light reflected from the bar code symbols by maintaining the light wand at an optimum angle with respect to the substrate material. An alignment tip may further be utilized with the light wand to ensure that the light wand is maintained at the optimum angle. The alignment tip includes a notched portion adapted to engage an edge of the substrate material so as to train an operator to maintain the optimum angle for specular light imaging.
A more complete understanding of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly.